The present invention relates to a temperature-compensated Zener-diode and a process for its production. The diode is to be able to operate under irradiation, such as neutron irradiation.
It is known that a diode is an electronic component constituted by a semiconductor material having an n region and a p region, the association of these two regions giving a pn junction.
The continuous line in FIG. 1 indicates the characteristic of a diode, i.e. the curve giving the electric current I as a function of the voltage V. The diode has two possible operating modes, depending on the direction of travel of the current passing through it.
Part A of the characteristic corresponds to forward biased operation, i.e. the current enters the diode by the n region.
In part A of the characteristic the voltage rises rapidly as a function of the current until it reaches a value V.sub.D, above which the voltage varies little. Generally voltage V.sub.D is between 0.5 and 1 V. In part B of the characteristic the current remains substantially zero until the voltage reaches a value V.sub.Z, called the avalanche or Zener voltage, above which the current rises very rapidly.
A temperature-compensated Zener diode or TCZ is a semiconductor component, whose function is to form a voltage reference, i.e. it guarantees a stable voltage no matter what the ambient conditions and in particular the temperature, supply variation, neutron irradiation, etc. Its function consists of opposing two pn junctions, one reverse-biased and the other forward-biased. This leads to a temperature-minimized voltage drift resulting from an adjustment between the positive drift or dV.sub.z /dT ratio of the reverse-biased junction or Zener junction and the negative drift or dV.sub.D /dT ratio of the forward-biased junction, the ratio depending little on the voltage V.sub.D, i.e. about -1.5/.degree. C. for a voltage V.sub.D of about 0.6 to 0.7 V. Conversely to obtain a variation dV.sub.z /dT of the same order of magnitude (but positive), it is necessary to have a value of the Zener voltage V.sub.z of about 5.5 to 5.6 V, leading to a total voltage of the temperature-compensated device of about 6.1 to 6.3 V.
Thus, all the construction technology of a temerature-compensated diode must be directed at providing two pn junctions having very different operating characteristics, but whereof the temperature drifts are very close to one another in absolute values. For information the temperature drift of a reverse-biased junction for V.sub.Z equal to 5.5 V is about 2.10.sup.-4 /.degree. C. The compensation by a forward biased junction must be able to bring this value to at the most 10.sup.-5 /.degree. C.
Among the hitherto known temperature-compensated Zener diode production processes one of them shown in FIG. 2 consists of producing pn junctions (or Zener diodes) by epitaxy, i.e. the p regions are deposited on a type n substrate by means of an iodine-based vector gas, followed by the annealing of the junctions. This procedure has the advantage of "burying" the active part of the pn junction a long way from surface interference leading to good reliability and reproducibility, so that high quality diodes are obtained. The main production parameters are then the resistivity of the initial substrate which is linked with the concentration of the doping particles for the N region and the annealing times and temperatures for the various operations.
The complete assembly of the TCZ diode is based on the aforementioned process and is of a monolithic nature, the two pn junctions are produced on the same type n substrate leading to an ideal thermal coupling when they are joined. For more details on the production of such a diode reference should be made to French Pat. No. 1,522,532 filed on Mar. 17, 1967 by Sescosem and entitled "Improvements to Zener diodes".
FIG. 2 shows such a TCZ diode, part 2 corresponding approximately to the forward-biased pn junction formed by P region 6 and N region 14 arrow I giving the direction of the current flowing through the diode and part 4 corresponding to the reverse-biased or Zener pn junction formed by N region 14 and P region 16. Region 6 is the buried or embedded active region of the pn junction. The diode is also provided with two conductive layers 8 placed on the two p regions from which lead connections 10 enabling the diode to be connected to different electrical circuits, and two insulating layers 12 deposited on part of the two n regions.
However, such a TCZ diode undergoes serious damage when exposed to neutron irradiation and in particular its characteristic curve is modified. These modifications are represented by dotted lines in FIG. 1.
It can be seen that the voltage V.sub.D decreases greatly, whilst the temperature coefficient dV.sub.D /dT increases in absolute values. Moreover the Zener voltage V.sub.Z increases, whereas the temperature coefficient dV.sub.Z /dT decreases. Thus, there are significant variations to the total reference voltage and to the temperature coefficient.
In simplified terms, the deterioration of the electrical characteristics of the diode results from the creation of defects or voids in the semiconductor material resulting from neutron bombardments and corresponding to a displacement of the atoms constituting the material. These defects tend to move and regroup with other impurities forming what are called complex defects and they modify the electrical charaacteristics of the diodes.